Chapter 4 Rocks And Minerals Different Types Of Rocks
Rocks are so common in nature that we come across them, every time, we visit a hill station in the form of the hard substance that makes up a mountain or while travelling on a train, we find these rocks lying near.
The railway tracks or at a riverside in the form of a rounded pebble or simply in heaps by the side of a road ready to be used in construction.
The earth’s crust is actually made up of various types of rocks that differ from one another in colour, structure, mode of formation, resistance to erosion, etc. The surface rocks are covered by soil or clay in most places.
Rocks are natural substances composed of solid crystals of different minerals that have been fused together to make solid lumps.
These may be classified into three major types- Igneous, sedimentary and metamorphic according to their mode of origin and appearance.
Read And Learn More: WBBSE Notes For Class 6 School Science
Chapter 4 Rocks And Minerals Igneous Rock
The word igneous comes from the Latin word ‘ignis’ meaning fire. This is a clue to the fact that the rocks were originally very hot.
It is believed that just after the earth was created, it was very hot and was made up of molten rocks. In the course of time, these molten rock materials cooled down slowly into solid rock.
Thus, the first hard rock formed on or under the earth’s crust is called igneous rock, also known as the primary rock.
Igneous rocks are formed by the cooling and solidification (hardening) of lava on the earth’s crust or magma under the earth’s crust.
Formation & types: Deep inside the earth due to increasing temperature and pressure the rocks remain in a molten state as magma. This magma rises to the surface through cracks in the crust.
While coming out it undergoes changes in temperature and pressure that cause it to cool, solidify and crystalize beneath the surface of the Earth’s crust.
These rocks are known as intrusive igneous rocks. Intrusive igneous rocks are very hard and often coarse-grained in texture.
Examples of these types of rock are granite and gabbro. When the magma comes out of the cracks of the crust or through the crater (vent) of a volcano it is called lava.
This lava cools and solidifies on the earth’s surface quickly to form another kind of igneous rock, known as extrusive igneous rock.
Extrusive igneous rock is fine-grained. Examples of this type include basalt and pumice.
Granite, basalt and pumice are the three commonest types of igneous rocks.
Granite is a light-coloured igneous rock with grains large enough to be visible to the unaided eye. Granite is widely used in most construction activities because of its strength.
Basalt is a fine-grained dark-coloured (usually black) rock of volcanic origin. They are the stones commonly seen near railway tracks.
Pumice stone is actually a volcanic rock which may or may not contain crystals. It is dark-coloured and denser.
It is usually ejected from a volcano. Pumice is formed when there is rapid cooling and depressurization. Holes are usually seen on the pumice stone.
These holes are formed when the dissolved gases of the hot magma escape in the air and the froth at the top of the hot magma quickly cools and solidifies.
Pumice stones are commonly used as abrasive materials in consumer products and aggregate in concrete mixtures.
Chapter 4 Rocks And Minerals Sedimentary Rock
Rocks that are formed through the layerwise deposition and solidification of sediments, especially sediments transported by water (rivers, lakes and oceans), glaciers and wind, over a long time span are called sedimentary rock.
Formation:
Sedimentary rocks are formed by the deposition and subsequent cementation of sediments, within the bodies of water like seas, lakes, etc.
The sediments are actually broken rock fragments derived from the primary igneous rocks.
These rocks were exposed to the action of the agents of erosion like rain, wind, rivers, glaciers etc. and the eroded materials were transported and deposited in layers over a long time, underwater.
When sedimentation continues the older rock layers become buried deeper. Over a long period due to the increase of heat and pressure of the overlying sediments, the underlying
Chapter 4 Rocks And Minerals Metamorphic Rock
Metamorphic rock is a type of rock that is formed when the igneous or sedimentary rocks transform or change physically and/or chemically, due to very high heat and pressure.
The upper layer of the earth is mainly formed of primary (igneous) and secondary (sedimentary) rocks.
The metamorphic rocks are found under the earth’s surface and their sediments get compacted and cemented and turn into sedimentary rocks.
We find petrified remains (fossils) of animals, plants and other microorganisms in the sedimentary rocks as these rocks were formed underwater.
Three common examples of sedimentary rocks are sandstone; limestone and shale.
Sedimentary rocks are deposited in layers as strata and they extensively (73%) cover the earth’s current land surface.
These rocks exist as a thin sheet over the earth’s crust consisting mainly of igneous and metamorphic rocks.
original character and appearance are greatly altered due to metamorphism.
Some common metamorphic rocks are gneiss transformed from granite, marble transformed from limestone, and slate metamorphosed from shale.
Metamorphic rocks are of two types-Foliated (or layered) and non-Foliated metamorphic rocks.
Chapter 4 Rocks And Minerals Minerals And Ore
In the previous chapter, you have learnt about metals, which are solid objects, hard, shiny, good conductors of heat and electricity, can be turned into sheets and their thin pieces can be easily bent.
Iron, copper, aluminium etc. from which nails, wires or utensils can be made are metals. Metals have played a very vital role in the progress of civilisation.
Now the obvious question is how do we get metals? The answer is that our earth is the main source of metals. We extract metals from the earth’s crust.
Earth’s crust is made up of aluminium (8.1%), iron (5%), calcium (3.6%), sodium (2.8%), potassium (2.6%)
What do these indicate? It shows that metals react in the open air to make new compounds.
From the above examples, it can be concluded that metals like iron or copper are found as compounds in nature and not in a pure state.
Likewise aluminium, zinc etc. are similar metals found as compounds.
Gold however does not react when exposed to air or water. It usually remains an element in nature.
Since metals do not occur freely in nature, they are found as compounds mixed with sand and soil, i.e. they occur as minerals.
A mineral is a naturally occurring solid. an inorganic, crystalline substance with a definite chemical composition.
They are formed naturally by geological processes. A mineral can be made of a single chemical element or more usually a compound. Minerals are most commonly associated with rocks.
Rocks may consist of one type of mineral or may be an aggregate of two or more different types of minerals.
When minerals are explored or mined, metals are separated from them. The process of separating metals from the minerals is known as metal extraction.
The type of mineral from which metal can be extracted easily and economically is known as its ore.
Examples: haematite (ferric oxide: Iron + Oxygen) is the mineral ore of iron, bauxite and magnesium (2.1%) among others.
Thus aluminium is the most abundant metal on this planet. It is now important to know, whether, these metals remain as elements or compounds in nature.
If you notice carefully you will observe that an iron object develops a yellowish-red coloured rust, if it remains exposed to air and moisture over a long time.
Again a greenish stain appears on copper utensils when it is left unused for a long time.
(Aluminium + Oxygen) is the mineral ore of aluminium and copper is extracted from its ore copper glance or chalcocite (copper sulphide: Copper + Sulphur).
If a metal has more than one mineral form, then it is not necessary that all of them are ores.
For every mineral deposit, there is a set of conditions, such as the level of concentration of the metal and the size of the deposit that, the deposit may be worked at a profit.
If a mineral deposit is sufficiently rich to be worked at a profit, it will then be called an ore deposit, otherwise not.
So, all ore deposits are mineral deposits, but the reverse is not true.
Chapter 4 Rocks And Minerals Alloys
Metals, after they are extracted from ores, look different from their ores. This is because a chemical change takes place in the process of metal extraction.
The use of steel, brass, bell metal, bronze, etc. in our daily lives is quite common nowadays. These metals are, however, not pure metals like iron, copper etc.
They are combinations of two or more metallic elements.
The homogeneous mixture or combination of different metals or metal and non-metal in a fixed ratio (especially to give strength) are called alloys.
For example, bronze is made up of copper and tin; iron and carbon are mixed to obtain steel; brass is formed when copper and zinc combine.
The two or more component elements that combine to form an alloy are inseparable, i.e. they cannot be readily separated by any physical means.
The major metallic component of the alloy is called the main metal or parent metal or base metal.
Today, more than 90% of metals used are in the form of alloys.
This is because alloys have many advantages, that a single metal does not have.
- Alloys are much stronger than a single metal,
- An alloy has the ability to withstand heavy weight,
- It does not readily respond to chemical reactions and
- Alloys are resistant to corrosion.
Alloys are not always a mixture of two or more metals but may contain one metal combined with other substances, non-metals etc.
An alloy is made by melting two or more elements together, one of which is a metal. When the mixture cools down, a solid substance called alloy is obtained.
Example: Iron mixed with carbon forms steel which is stronger than iron and can support huge weight.
Chromium mixed with iron forms stainless steel which is resistant to corrosion and hardly responds to chemical reactions.
Besides these, gold, duralumin, solder, the metal used to make the electric fuse, etc. are common alloys.
Chapter 4 Rocks And Minerals Fossils
The pictures show the remains of a fish-
- The shell of a sea snail
- And the remains of a sea star
We find all these remains engraved in rocks. Once all these were living but with the passage of time their dead remains got petrified or buried in rocks.
These are called fossils. luas After undergoing different changes under the earth over a period of millions of years, the remains of dead animals and plants (living beings of the geologic past) got transformed into rocks.
These rock-like remains of once-living things are called fossils. The preserved impression of an extinct organism’s body parts is also called a fossil.
The trapped body of a dead insect inside the resin of a tree is also called a fossil.
It is really interesting to know how the dead bodies of these animals came inside the rock and got preserved in a hard, petrified form. This process is known as fossilization.
Fossilization:
Millions of years ago the fish, the snail and the sea star were all alive. After the death of these animals, their bodies got buried under water or under the surface of the earth.
The soft fleshy parts of their bodies decayed in the course of time. Soon after the initial decay process, the remains of their bodies got covered with sediments.
The remains, after undergoing a variety of physical and chemical changes over a long span of time, got transformed into rocks.
The impressions or rocky remains of once-living organisms are thus converted into fossils through fossilization or petrification.
Fossils found in sedimentary rocks, act as useful tools for dating the rocks. They also show us the long history of life and the past.
Like animal fossils, plant fossils are commonly seen on coal formed many million years ago. At that time our earth was covered with forests where varieties of plants, ferns, seeds, mosses etc. grew.
The plants grew and eventually died and fell into the shallow waters. New plants grew in their places and later died, got uprooted and fell on the previous layer of decaying plants.
This process of plant growth and their death added layers Raad of decaying plants in the swampy water. Soil, mud and other sediments subsequently covered the decaying plants.
They were gradually pushed underground by the weight. of the overlying layers. The weight of the top layers and heat and pressure caused chemical and physical changes in the plant layers.
As a result, rich coal deposits were formed with impressions of leaves left behind on them.
Chapter 4 Rocks And Minerals Fossil Fuels
The materials such as coal, gas or oil that are burnt to produce heat and power are called fuels.
Coal, gas and oil are obtained from the remnants or dead remains of plants and animals that lived a long time ago.
These dead remains had undergone changes for millions of years and were transformed into fossils.
The trapped solar energy inside the fossils (in the form of chemical energy) produces heat and light energy when these fossilized remains are burnt.
Hence, coal, petroleum (oil) and natural gas are known as fossil fuels.
There are several other types of fuels like wood, paper, hay etc., which do not take millions of years to form. Hence, they are not called fossil fuels.
The pictures show the processes of the formation of petroleum. The pictures show that about 300 million years ago marine animals and plants died and were eventually buried under layers of sediments (silt, sand etc.).
The plants and animals got decomposed partially. Immense heat and pressure acted on the decomposing materials, which resulted in the formation of sedimentary rocks containing petroleum and natural gas.
Finally, we drill through the rocks and sediments to discover petroleum accumulations.
Chapter 4 Rocks And Minerals Uses of Fossil Fuel
Coal is perhaps the most widely used fossil fuel It has various uses. The major uses are outlined below:
1. As fuel- In the past coal was mainly used to provide heat and raise steam. You must have heard of steam engines or steamships which used coal as a fuel.
Clay ovens used in many households, even today, in rural areas depend on coal for generating fire. Nowadays, the use of coal in both these fields is gradually being replaced by oil or electricity.
2. Electric power- Coal is still widely used in thermal generators (thermal power plants) to produce electricity. Coal is burnt to generate heat which in turn produces electricity.
3. As coke in the Iron and steel industry-coal is still irreplaceable in the smelting (heating and melting) of iron and steel. Coal can be used to make metallurgical coke for use in blast furnaces.
Coke is produced when coal is heated at a high temperature (about 1100°C) in the absence of oxygen. Coke is essential in the process of extraction of pig iron or hot metal.
Coal tar is also produced from coal as a by-product of coke production. Organic compounds having active hydrogen are separated from tar. The Coke oven gas is used as fuel after it is refined.
Chapter 4 Rocks And Minerals Uses of Petroleum
Oil or petroleum has a wide range of uses as a fuel, as a lubricant, as a raw material for a variety of products etc. The major use of petroleum is in the field of transportation as a fuel.
1. Transport sector :
Petrol (gasoline) and diesel oil are mostly used to drive vehicles like cars, buses, motorcycles, lorries, small boats, military tanks etc.
2. Domestic and commercial cooking gas:
LPG (Liquefied Petroleum Gas) cylinder is used in our kitchen as a common and clean household fuel.
3. Petroleum products:
Petroleum or rock oil is a naturally occurring yellow-to-black liquid which is actually a sticky mixture of various organic compounds, water and soil.
This unprocessed crude oil is refined in a refinery and is made free of water, soil or other impurities to produce various types of fuels. This process is known as the refining of petroleum.
A large number of consumer products like gasoline (petrol), kerosene, asphalt, bitumen or tar etc.
are produced from refined petroleum. Gases such as butane and propane are also produced. LPG cylinders mainly contain liquid propane, butane, propylene, butylene etc.
Chapter 4 Rocks And Minerals Uses Of Natural Gas
Natural gas usually occurs in the uppermost part of an oil trap. The principal constituents of natural gas are a mixture of gaseous hydrocarbons.
Of these methane alone constitutes about 80 to 90 per cent while the other gases include, ethane, propane and butane.
Compressed Natural Gas or CNG is made when purified natural gas is compressed (put under pressure) and stored in a cylinder and used as fuel.
In our country as well as in many other countries CNG is used to run public transport, especially buses, because it Besides its use as fuel, petroleum has become the raw material of a number of chemical industries.
Fertilisers, insecticides and other chemicals for agricultural use; solvents and detergents; plastics, lubricant oil, paint etc.
Are also made from petroleum causing less pollution than other commonly used fuels like diesel or petrol.
In some parts of India, CNG is supplied through underground pipelines to homes and factories to be used as fuel.
CNG is also used in power generation.